1. Field of the Invention
This invention relates to an ion source, and more particularly to a microwave plasma ion source suitable for high current implanters.
2. Description of the Prior Art
Implanters can be broadly classified into the low current type (10 .mu.A-below 1 mA) and the high current type (1 mA or above). This is because required impurity doses differ depending on the semiconductor devices to be manufactured, and the range is as wide as 10.sup.12 -10.sup.16 ions/cm.sup.2. Since low current type implanters are easier to fabricate, most of the implanters presently in operation are of the low current type. Accordingly, the fields of application of the implanters have been limited to those where the doses are comparatively small (for example, the channels of MOS transistors and the bases of bipolar transistors). Recently, however, there has arisen a demand to form emitters of bipolar transistors and sources and drains of MOS transistors, where the doses are large, also by ion implantation. High current type implanters are required to meet this demand. In general, an implanter is constructed of an ion source, a mass separator, and a target chamber. Whether the implanter is of the high or the low current type depends upon the performance of the ion source used. At present, there are two sorts of apparatuses capable of implantation on the order of 1 mA. One is an apparatus employing a thermionic filament type ion source, in which low voltage arc discharge is caused by thermions from a filament parallel to an ion extracting slit, and ions are extracted in the form of a slit-shaped beam from the plasma produced by the discharge; the other is an apparatus employing a microwave plasma ion source, which will be described in detail below. The former has been presented, for example, in "The PR-30 Ion Implantation System" at the 14th Symposium on Electron, Ion and Photo Beam Technology, May 1977, while the latter is described in U.S. Pat. No. 4,058,748 issued Nov. 15, 1977. The ion sources of these two types will now be compared. The lifetime of the former is determined by the lifetime of the filament used, and is ordinarily several hours to ten or fifteen hours. In contrast, the latter has a very long lifetime because it does not include consumable parts, such as the cathode in the former. However, when PH.sub.3 gas (for P.sup.+ ions), AsH.sub.3 gas (for As.sup.+ ions) or the like is used as the gas to be ionized, the dissociated P or As gradually deposits on the surface parts of the electrodes, since the electrodes are in contact with the discharge chamber. The deposits close up the exit for the ion beam, becoming a cause of abnormal discharge within the discharge chamber. As a result, the ion beam becomes unstable after about 10-20 hours of operation, which is inconvenient.